1. Field of the Invention
The present invention relates to a screwdriver used for a screw tightener and to a screw suitably tightened by the screwdriver, and particularly to the structure of a screwdriver used for a screw tightener adapted to turn a screw using a power and to the structure of a screw suitably tightened by the screwdriver.
2. Description of the Related Art
A known screwdriver used for a screw tightener automatically rotated using a power source such as an air motor is shown in FIGS. 19 and 20. In these figures, reference numeral 1 designates a screwdriver having at its tip portion a blade 5 to be fitted in a recess 4 formed in a head 3 of a screw 2 shown in FIGS. 21 and 22. The tip portion of the blade 5 is sharpened in a truncated cone shape, and outer peripheral surfaces 7 of flat plate portions 6 forming the blade 5 are formed to constitute a peripheral surface of the cone.
As the recess 4 formed in the head 3 of the screw 2, there is known a type in which an intersecting portion 8 between two adjacent ones of grooves 4A and 4B constituting the recess 4 is formed into a round shape as shown in FIG. 23 or a type in which an intersecting portion 9 is formed into an angled shape as shown in FIG. 24.
Incidently, of recent screw tighteners using an electric or air motor as a drive source, one type is known in which the head 3 of the screw 2 is set in such a manner as to be held by the tip portion of the blade 5 of the screwdriver 1.
The screw 2 is screwed in an object (for example, a construction board such as a gypsum board) by such a screw tightener as follows: namely, the screw 2 is set in the screw tightener with the tip portion thereof directed to the construction board; a trigger of the screw tightener is actuated to make the screwdriver 1 collide with the head 3 of the screw 2 for fitting the blade 5 of the screwdriver 1 in the recess 4, whereby the tip portion of the screw 2 is driven in the construction board; and the screwdriver 1 is rotated to screw the screw 2 in the construction board. In this case, when the screw 2 is driven in the construction board by the initial impact, a large portion of the screw 2 is buried in the construction board, and the remaining screw-in portion of the screw becomes short. Accordingly, the related art screw tightener has an advantage in making short a working time required to tighten the screw by the screw tightener.
The related art screwdriver used for a screw tightener, however, has the following problem. When the screw 2 is screwed in an object by the screw tightener, the outer peripheral surfaces 7 of the blade 5 of the screwdriver 1 are brought in contact with the intersecting portions 8 or intersecting portions 9 forming the recess 4 of the screw 2. This may cause an inconvenience that the screwdriver 1 is rotated in a state in which the blade 5 is not perfectly fitted in the recess 4. In such a state, the screwdriver 1 rotating at a high speed cannot be fitted in the recess 4 of the screw 2, with a result that the tightening work is completed in a "head floating" state in which the screw-in portion of the screw 2 is floated from the construction board, thereby failing to obtain a sufficient tightening force. Also, in such a state, the intersecting portions 8 or 9 forming the recess 4 may be damaged, so that it takes a lot of time to draw the screw 2, thereby degrading workability, and further, the flat plate portions 6 of the screwdriver 1 may be damaged.
In particular, if an apex angle of each of the outer peripheral surfaces 7, formed to constitute the peripheral surface of the cone, of the flat plate portions 6 forming the blade 5 is large, there occurs a problem that since the radius of the outer peripheral surface 7 is kept constant, a probability becomes large in which the blade 5 is not fitted in the recess 4 when the outer peripheral surfaces 7 are brought in contact with the intersecting portions 8 or 9.
The related art screwdriver 1 has a further problem that since the screwdriver 1 cannot firmly hold the screw 2, the screw 2 may be tilted in relation to the construction board. If the screwdriver 1 is rotated in such a state, a load necessary for screwing the screw 2 in the construction board becomes large to a value over the capacity of the screw tightener, and at the worst case, it becomes impossible to continue the screwing work.
Even if the screw 2 can be fortunately screwed in the construction board, there may occur an inconvenience that the head 3 of the screw 2 is tilted, with a result that the surface of the construction board is broken by a portion, on the sunk side, of a circumferential edge of the head 3, to thereby reduce the proof stress at a junction between the screw 2 and the construction board.